The electrodialytic processes as disclosed in my U.S. Pat. No. 4,636,288 provide a broadly applicable method for the electrodialytic conversion of multivalent metal salts in aqueous solutions into the acid or halogen of the salt anion and the hydroxide or insoluble salt of the metal cation. These salt splitting processes comprise the electrotransport of multivalent metal cations through a cation permeable membrane into an aqueous solution or catholyte containing a soluble salt of an acid which acid in a one normal solution would have a pH no greater than three and forms a water soluble salt of a multivalent metal cation and agents that insolubilize or ionically immobilize multivalent metal cations. The aqueous solutions receiving the multivalent metal cations are generally alkaline, having a pH greater than seven. These processes are used broadly for the purification and restoration of solutions containing chromic acid that are used for electroplating, anodizing, chromating and etching metals and plastics. At times trace quantities of chromate ions enter the alkaline catholyte and it is necessary to remove the chromate ions or to reduce the chromate ion to chromium.sup.+3 and to precipitate chromium.sup.+3 hydroxide to meet requirements for disposal of the alkaline catholyte. It is known that chromate ions can be reduced in acidic solutions, having a pH of about three, with reducing agents such as sulfur dioxide, sodium bisulfite and ferrous sulfate. The reactions are usually carried out in excess acid to complete the reactions and the reaction solution is then made alkaline to precipitate the chromium.sup.+3. It would be preferable that the chromate ions in the alkaline electrolyte be converted in the electrolyte without acidification and the electrolyte could be used continuously. It is an object of the instant invention to provide a method for conversion of chromate to chromium.sup.+3 ions in the alkaline electrolyte.
Oxidation and reduction reactions have been researched extensively and reported in literature. These reactions must always occur simultaneously. The element, compound or ion which has been reduced gains electrons from some element, compound or ion which is thereby oxidized. Every reaction involving an exchange of electrical charges is an oxidation-reduction reaction. It is not strictly accurate to describe the change merely as an oxidation or reduction reaction but often for simplicity reactions which involve oxidation-reduction are described as oxidation if the accompanying reduction is of subordinate interest. Hydrogen occupies an intermediate position in the electromotive series of elements. Consequently it acts as an oxidizing agent towards the metals which precede it and a reducing agent towards those which follow it in the series. The terms oxidizing agent and reducing agent depend upon the nature of the substance and the reaction environment with which it is brought into contact. It is known that pH of the reaction environment is an important variable in oxidation-reduction and most chemical reactions.
It is an object of this invention to provide a process for forming and maintaining with electricity a reaction environment that is a continuous liquid part acidic and part alkaline for carrying out oxidation-reductions and other chemical reactions.
Electrodialysis is a well know art (See U.S. Pat. Nos. 4,325,792; 4,439,293; 4,636,288; 4,652,351 and 4,684,453 the disclosures of which are incorporated by reference). Electrodialysis is the transport of ions through ion permeable membranes as a result of an electrical driving force. The process is commonly carried out in an electrochemical cell having a cathode compartment containing a cathode and a catholyte and an anolyte compartment containing an anode and anolyte, the catholyte and anolyte compartments being separated by ion permeable membranes. Electrodialysis can be carried out in cells containing more than two compartments and more than two electrolytes. When electricity is passed through an electrodialytic cell, cations tend to go to the cell cathode and anions to the cell anode. Oxidation occurs at the cell anode and reduction at the cell cathode. Water is electrolyzed to hydrogen ions and oxygen and hydroxyl ions and hydrogen. The electrodialytic conversion of multivalent metal salts is disclosed in U.S. Pat. No. 4,636,288.